1. Field of the Invention
The present invention relates to a pneumatic tire provided with a directional or unidirectional tread. In the directional tread tire, the tire is mounted on a rim under consideration of the directional pattern formed on the crown portion of the tire tread.
2. Description of the Prior Art
Recently, pneumatic tires provided with a directional tread have been proposed in order to improve drainage performance, in particular at high speed travelling.
In the prior-art pneumatic tires provided with a directional tread, the tread is divided into a number of blocks by a plurality of circumferential grooves arranged in parallel to the tire circumferential direction, and a plurality of U-shaped cross-section transversal grooves arranged at regular intervals in a herringbone pattern along the tire circumference in such a way as to extend from the central tread area to both the tread side areas at an inclination angle with respect to the tread circumference direction.
In the prior-art tire as shown in FIG. 2(B), however, the cross section of each of the transversal grooves is formed into a U-shape in such a way that the inclination angles .theta..sub.1 and .theta..sub.2 of both the groove walls with respect to the tire radial direction and the radii R.sub.1 and R.sub.2 of the curvature of groove bottom corners 2A and 2B are formed equal to each other in symmetry on both the front and rear sides along the tire rotating direction. In other words, in the prior-art tire as described above, the tire rotating direction is determined in such a way that the tread central portions of the transversal grooves are first brought into contact with the ground and thereafter both the tread side portions thereof are brought into contact with the ground to improve drainage performance in dependence upon the directional herringbone pattern.
In the above-mentioned prior-art tire, although there exists no problem at low and medium speed, however, when the tire is rotated at high speed of 250 km/h or more, there exists a problem in that cracks are readily produced at the groove bottom corners of the transversal grooves and therefore the blocks 3 are easily broken off, thus reducing the tire life. This is because a large shock or load is first applied to the groove bottom corner 2A of the step-out side (first brought into contact with the ground) along the circumferential direction of the blocks 3, so that a large strain is produced at the groove bottom corner 2B of the kick-out side (afterward brought into contact with the ground) when seen in the cross section of the transversal groove 2 as shown in FIG. 2(B), thus reducing the tire life.